Interpretive Summary: Whereas much is known about relationships between rations, milk production, manure excretion, and environmental risks of confinement dairy production, much less is known about these relationships on grazing-based farms. The objectives of this study were to determine relationships on grazing-based farms (1) between feed nitrogen intake, milk production, milk urea nitrogen, feed nitrogen use efficiency, and excreted manure nitrogen, and (2) between feed phosphorus intake, fecal phosphorus concentrations, and excreted manure phosphorus. Results showed that the relationships on grazing-based farms were different than those on confinement-based farms. This information will help producers and consultants monitor and evaluate feed use efficiency and the economic and environmental impacts of nutrient use on both confinement and grazing-based dairy farms.

Technical Abstract:
Whereas much is known about relationships between rations, milk production, manure excretion, and environmental risks of confinement dairy production, much less is known about these relationships on grazing-based farms. The objectives of this study were to determine relationships on grazing-based farms (1) between feed N intake (NI), milk production, milk urea N (MUN), feed N use efficiency (FNUE), and excreted manure N (ExN); and (2) between feed phosphorus (P) intake (PI), fecal P (FP) concentrations, and excreted manure P (ExP). An additional objective was to assess correspondence between feed, milk and manure relationships established on confinement dairy farms to those on grazing-based dairy farms. Four dairy farms located south eastern Australia were visited during autumn and spring, 2008. Information was gathered on feed practices for six cows selected randomly from each of high, medium and low producing cow groups on each farm. Samples of each ration component, and milk and feces from each cow were taken. Total ration dry matter intake (DMI) was calculated as the sum of feed supplements (silage, hay, by-products, grain, concentrates) offered manually plus the amount of pasture estimated indirectly using a bio-energetic approach. Each farm offered a similar basal ration with varying levels of barley grain (0.5 to 8.8 kg cow-1 d-1) or concentrate (0.8 to 9.4 kg cow-1 d-1).Although milk production was greater during spring than autumn, milk response (2.23 L kg-1 DMI) was similar during both seasons. Milk responses to NI were greater during spring (63 mL g-1) than during autumn (55 mL g-1), and milk response to supplemental grain or concentrate were variable. Ration CP concentrations during spring ranged from 183 to 248 g kg-1, or 11% to 50% greater than recommendations for high producing cows on confinement farms. As ration CP increased FNUE declined. In contrast to findings on confinement farms, no significant relationships were found between ration CP and MUN, or between MUN, parity, or milking frequency. NI and DMI provided the best predictors of ExN, and PI provided the most accurate prediction of ExP. Unlike findings on confinement farms, no relationships were found between ration P and fecal P. A significant positive relationship between total P in feces and HCl-extractable P in feces indicates that ration P (range of 4.4 to 6.4 g kg-1) on the study farms exceeded cow requirements.